For those still on Leopard, I'd highly recommend Bodega.
Has a lot of apps for sale there and has been doing what the App Store is offering for awhile now.
Keeps track of purchase codes for apps purchased thru them.
Goes through your entire Applications folder to keep track of your previously installed apps to find updates for them. This is where the App Store needs to go, or there is still a place for Bodega on your system.

I have not bought anything direct through this. I primarily use it for it's auto updating feature of all my apps. It's a great tool for that, especially for those like my wife that never goes and checks for updates on anything.

Download and one ready to install pic:

I cannot be held responsible for the things that come out of my mouth.
In the Windows world, most everything folks don't understand is called a virus.